132 DENSITY OF THE ATMOSPHERE. [SECT. xv. 



increasing arithmetical progression that is, if they increase 

 by equal quantities, as by a foot or a mile, the densities of 

 the strata of air, or the heights of the barometer which are 

 proportionate to them, will decrease in geometrical pro- 

 gression. For example, at the level of the sea if the mean 

 height of the barometer be 29 '922 inches, at the height of 

 18,000 feet, it will be 14'961 inches, or one-half as great ; 

 at the height of 36,000 feet, it will be one-fourth as great ; 

 at 54,000 feet, it will be one-eighth, and so on, which 

 affords a method of measuring the heights of mountains with 

 considerable accuracy, and would be very simple, if the 

 decrease in the density of the air were exactly according to 

 the preceding law. But it is modified by several circum- 

 stances, and chiefly by changes of temperature, because heat 

 dilates the air and cold contracts it, varying ,fo of the whole 

 bulk when at 32, for every degree of Fahrenheit's ther- 

 mometer. Experience shows that the heat of the air de- 

 creases as the height above the surface of the earth increases. 

 And it appears from recent investigations that the mean 

 temperature of space is 58 below the zero point of Fahren- 

 heit, which would probably be the temperature of the sur- 

 face of the earth also were it not for the non-conducting 

 power of the air, whence it is enabled to retain the heat of 

 the sun's rays, which the earth imbibes and radiates in all 

 directions. The decrease in heat is very irregular ; each 

 authority gives a different estimate ; probably because the 

 decrease varies with the latitude as well as the height, and 

 something is due also to local circumstances. But, from the 

 mean of five different statements, it seems to be about one 

 degree for every 334 feet, which is the cause of the severe 

 cold and eternal snows on the summits of the Alpine 

 chains. Of the various methods of computing heights from 

 barometrical measurements, that of Mr. Ivory has the ad- 

 vantage of combining accuracy with the greatest simplicity. 

 Indeed, the accuracy with which the heights of mountains 

 can be obtained by this method is very remarkable. Captain 



